Generally, polyester resins are prepared by reacting aromatic and aliphatic dicarboxylic acids with diol compounds. Polyester resins have excellent physical and chemical properties, are highly soluble in general solvents, are highly flexible, have good adhesion to a broad range of materials and good coating workability, etc. Due to these advantages, polyester resins are used in various applications, such as for fibers, films, and adhesives.
However, previously known polyester resins have poor resistance to chemicals, for example, surfactants, to which humans are frequently exposed in practical life, have colors and appearance properties unsuitable for use in particular applications, and have insufficient impact strength. Attempts to solve such problems of polyesters have been made through many methods, for example, methods including adding various starting materials, monomers, or additives in the course of synthesis, or methods including mixing with other resins. Particularly, previous synthetic fibers or synthetic resin products obtained from polyesters tend to yellow in appearance. In attempts to hide or counteract such appearance properties, methods associated with the use of additives such as cobalt-based colorants have been generally used.
It is known that in the synthesis of a polyester resin, the degree of polymerization of the final polyester resin or the proportions of raw materials present in the main chain of the resin vary depending on the reactivity of the raw materials during esterification or transesterification and on the degree of vaporization of the raw materials during polycondensation. Particularly, when it is intended to synthesize a polyester resin using a secondary or tertiary alcohol as a diol component, a sufficiently long reaction time is required or a very low reaction yield is obtained.
A method for synthesizing a polyester resin using an alcohol such as 1,4-cyclohexanedimethanol or isosorbide as a reactant has been known in recent years. According to this method, however, the use of the secondary alcohol makes it difficult to achieve a high reaction yield and a markedly improved degree of polymerization of the resin. A polyester resin synthesized by such a method fails to sufficiently ensure physical properties such as heat resistance, chemical resistance, and impact resistance above optimum levels, or does not have appropriate appearance properties in terms of color, transparency, etc.
Thus, there is a need to develop a method for preparing a polyester resin with improved physical properties and excellent appearance properties, by which the efficiency of the polymerization reaction and the residual rate of raw materials in the final product can be increased.